1. Field of the Invention
This invention relates to lubricative coating of polymeric monofilaments. More particularly, the present invention involves applying a polymeric coating finish that provides excellent lubricative characteristics to a polymeric monofilament.
2. Description of the Prior Art
Weedtrimmers of the line cutting type use flailing movement of flexible filament, which usually is made of monofilamentary polymeric material, to cut the grass or weeds. Polyamides, particularly nylon 6, nylon 6,6 and copolymers thereof, are the most typically used polymeric materials for the monofilament in such line applications.
Such a conventional weedtrimmer has a rotable driver head from which one or more of the flexible cutting filaments extends out. The rotable head contains a storage spool and metering device which feed from the spool a length of filament line as the filament line abrades away or breaks away in use. U.S. Pat. No. 4,316,325 and U.S. Pat. No. 4,651,422 describe such weedtrimmers and their parts. A conventional and most widely used mechanism to replenish the cutting filament is known as the "bump head" in the art. The mechanism allows an incremental length of the cutting filament to be replenished when tapped sharply on the ground. However, when the driver head rotates at a high speed, the centrifugal force of the flailing filament tends to tighten and knot the reserve filament on the spool. Furthermore, the filament on the spool is subject to high heat and constant vibration resulting from the high speed rotation and the constant impact upon the cutting filament outside the spool. Frequently, the heat and vibration on the spool create an environment hot and severe enough to subject the filament on the spool to be vibrationally welded together. Such knotting and welding of the filament impede the release of the reserve filament from the spool.
It is known in the weedtrimmer industry that lubricating the filament can alleviate the filament knotting and welding problems. However, utilizing a conventional lubricant may not be economically desirable. The application of conventional lubricants has been known to adversely affect the physical integrity of the filament polymer or require additional post-extrusion treatment processes. Also, it is economical and a common practice in the industry to recycle and reprocess substandard filaments since the filament manufacturing process produces amounts of substandard quality filaments that does not meet desired dimensional specifications. Consequently, it is desirable to have a coating material that will not only provide desired lubricity but will also not adversely affect the physical properties of the filament, especially when reprocessed. It is also desirable to have a lubricative coating process that does not require substantial modification of the current filament extrusion process.
There are three basic steps in the process of manufacturing the monofilament: extrusion, orientation, and relaxation. Polymer resin pellets are extruded as continuous strands of monofilament with an extruder; the resulting filaments are stretched longitudinally to orient the filament polymer in order to increase the strength of the filament; subsequently, the oriented filaments are relaxed to relieve the strain created from the orientation process. Each manufacturing step is preceded by heating and followed by cooling. Usually, heating is accomplished by passing polymeric resins or extruded strands, depending on the manufacturing step, through at least one heating oven, and cooling is accomplished by passing the strands through at least one water quench bath.
It is known in the weedtrimmer industry to employ water soluble organic and inorganic salts, such as triethanolamine, as lubricative coating materials for the weedtrimmer cutting filament. The addition of triethanolamine to the last quench bath that follows the relaxation process apparently provides satisfactory lubricity without requiring any modification of the manufacturing process. However, the filament coated with triethanolamine cannot be reprocessed since the strongly alkaline triethanolamine coating adversely interacts with the polymer of the filament during reprocessing (i.e., remelting). The adverse effect of triethanolamine creates bubbles and voids in the resulting monofilament, rendering the filament unsuitably frail.
The practice of overfinishing polymeric fiber to obtain lubricity is well known in the textile industry. However, the fiber overfinishing technology, known in the art as the spin finish or overspray, is not useful in coating the weedtrimmer cutting filaments. The spin finish lubricants, which are designed to be scourable to avoid interference with dyeing and other subsequent textile processing steps, are not suitable as polymeric monofilament coating materials since they do not form permanent coatings. A comprehensive list and characteristics of different spin finish lubricants are described in the article by Redston et al, Textile Research Journal, Vol. 43, 325-335, 1973.
The textile industry utilizes other types of finishes that are more permanent in nature although they may not be finishes to achieve lubricity. Such permanent finishes are applied to fabrics at the final stages of manufacturing in order to improve abrasion resistance, crease resistance, water repellency, and the like. In U.S. Pat. No. 2,426,770, Grim discloses a permanent water repellent and crease resistant finish that requires a heat polymerizing process. In U.S. Pat. No. 4,800,117 to Marshall (Marshall is one of the co-inventors herein), there is disclosed an abrasion resistant finish for multifilament yarns in which an melamine wax is used as one of the components of the finishing compound. The multifilament yarn finish taught in their patent also requires an additional heat polymerizing step after the finishing compound is applied to multifilament yarns.
It would be desirable to provide a coating material for monofilaments which exhibits the necessary lubricity, does not interfere in the reprocessibility of the monofilament, and does not require additional post-coating processes. It would also be desirable to provide a process for producing such monofilament, especially for use in weedtrimmers.